FIG. 1 is a simplified side cross-sectional view of a portion of a field emission display 10 including a faceplate 20 and a baseplate 21 in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate 20 includes a transparent viewing screen 22, a transparent conductive layer 24 and a cathodoluminescent layer 26. The transparent viewing screen 22 supports the layers 24 and 26, acts as a viewing surface and as a wall for a hermetically sealed package formed between the viewing screen 22 and the baseplate 21. The viewing screen 22 may be formed from glass. The transparent conductive layer 24 may be formed from indium tin oxide. The cathodoluminescent layer 26 may be segmented into localized portions. In a conventional monochrome display 10, each localized portion of the cathodoluminescent layer 26 forms one pixel of the monochrome display 10. Also, in a conventional color display 10, each localized portion of the cathodoluminescent layer 26 forms a green, red or blue sub-pixel of the color display 10. Materials useful as cathodoluminescent materials in the cathodoluminescent layer 26 include Y.sub.2 O.sub.3 :Eu (red, phosphor P-56), Y.sub.3 (A1, Ga).sub.5 O.sub.12 :Tb (green, phosphor P-53) and Y.sub.2 (SiO.sub.5):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda PA or from Nichia of Japan.
The baseplate 21 includes emitters 30 formed on a planar surface of a substrate 32 that is preferably a semiconductor material such as silicon. The substrate 32 is coated with a dielectric layer 34. In one embodiment, this is effected by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer 34 is formed to have a thickness that is approximately equal to or just less than a height of the emitters 30. This thickness is on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid 38 is formed on the dielectric layer 34. The extraction grid 38 may be formed, for example, as a thin layer of polysilicon. An opening 40 is created in the extraction grid 38 having a radius that is also approximately the separation of the extraction grid 38 from the tip of the emitter 30. The radius of the opening 40 may be about 0.4 microns, although larger or smaller openings 40 may also be employed.
In operation, the extraction grid 38 is biased to a voltage on the order of 100 volts, although higher or lower voltages may be used, while the substrate 32 is maintained at a voltage of about zero volts. Signals coupled to the emitters 30 allow electrons to flow to the emitter 30. Intense electrical fields between the emitter 30 and the extraction grid 38 cause emission of electrons from the emitter 30.
A larger positive voltage, ranging up to as much as 5,000 volts or more but usually 2,500 volts or less, is applied to the faceplate 20 via the transparent conductive layer 24. The electrons emitted from the emitter 30 are accelerated to the faceplate 20 by this voltage and strike the cathodoluminescent layer 26. This causes light emission in selected areas, i.e., those areas opposite the emitters 30, and forms luminous images such as text, pictures and the like.
Electrons emitted from each emitter 30 in a conventional field emission display 10 tend to spread out as the electrons travel from the emitter 30 to the cathodoluminescent layer 26 on the faceplate 20. If the electron emission spreads out too far, it will impact on more than one localized portion of the cathodoluminescent layer 26 of the field emission display 10. This phenomenon is known as "bleedover." The likelihood that bleedover may occur is exacerbated by any misalignment between the localized portions of the cathodoluminescent layer 26 and their associated sets of emitters 30.
When the electron emission from an emitter 30 associated with a first localized portion of the cathodoluminescent layer 26 also impacts on a second localized portion of the cathodoluminescent layer 26, both the first and second localized portions of the cathodoluminescent layer 26 emit light. As a result, the first pixel or sub-pixel uniquely associated with the first localized portion of the cathodoluminescent layer 26 correctly turns on, and a second pixel or sub-pixel uniquely associated with the second localized portion of the cathodoluminescent layer 26 incorrectly turns on. In a color field emission display 10, this can cause purple light to be emitted from a blue sub-pixel and a red sub-pixel together when only red light from the red sub-pixel was desired. As a result, a degraded image is formed on the faceplate 20 of the field emission display 10.
In a monochrome field emission display 10, color distortion does not occur, but the resolution of the image formed on the faceplate 20 is reduced by bleedover. In conventional field emission displays 10, bleedover is alleviated in several ways. A relatively high anode voltage V.sub.a may be applied to the transparent conductive layer 24 of the conventional field emission display 10, so that the electrons emitted from the emitters 30 are strongly accelerated to the faceplate 20. As a result, the electron emissions spread out less as they travel from the emitters 30 to the faceplate 20. A relatively small gap between the faceplate 20 and the baseplate 21 may be used, again reducing opportunity for spreading of the emitted electrons. However, it has been found that these are impractical solutions because too high a voltage applied between the transparent conductive layer 24 and the baseplate 21, or too small a gap between the faceplate 20 and the baseplate 21 may cause arcing.
Another way in which bleedover is reduced in conventional field emission displays 10 is by spacing the localized portions of the cathodoluminescent layer 26 relatively far apart. This is possible because of the relatively low display resolution provided by conventional field emission displays 10. As a result, the electron emissions impact on the correct localized portion of the cathodoluminescent layer 26.
Another approach to controlling the spatial spread of electrons emitted from a group of the emitters 30 is to surround the area emitting the electrons with a focusing electrode (not illustrated in FIG. 1). This allows increased control over the spatial distribution of the emitted electrons via control of the voltage applied to the focusing electrode, which in turn provides increased resolution for the resulting image. One such approach, where each focusing element serves many emitters, is described in U.S. Pat. No. 5,528,103, entitled "Field Emitter With Focusing Ridges Situated To Sides Of Gate", issued to Spindt et al.
There are several disadvantages to these prior art approaches. In most prior art approaches, the focusing electrode is biased by a voltage source that is independent of other bias voltage sources associated with the emitter 30. As a result, the use of a focusing electrode generally requires another bias voltage source to bias the focusing electrode. This, in turn, leads to problems due to variations in turn on voltage from one emitter 30 to another when a single bias voltage is applied for several focusing electrodes. When a group of emitters 30 are all affected by a single focusing electrode, some of the emitters 30 may exhibit a turn on voltage that differs from that exhibited by other emitters 30. The effect that the focusing electrode has on the electrons emitted from each of these emitters 30 will differ. Additionally, some of the current through the emitter 30 will be collected by the focusing electrode. This complicates the relationship between the emitter current and light emission because some of the current through the emitter 30 is diverted from the faceplate 20 by the focusing electrode. Further, the effects of the focusing electrode are different for emitters 30 that are closer to the focusing electrode than for emitters 30 that are farther away from the focusing electrode. The lack of control over the amount of light emitted in response to a known emitter current results in poorer imaging characteristics for the display 10.
The problem of bleedover is exacerbated by the trend to higher solution field emission displays 10. High resolution field emission displays use fewer emitters 30 per pixel or sub-pixel. This arises for several reasons, one of which is that a smaller pixel or sub-pixel subtends a smaller area in which the emitters 30 can be provided. As display engineers attempt to increase the display resolution of conventional field emission displays 10, the localized portions of the cathodoluminescent layer 26 are necessarily crowded closer together. As a result, each emitter 30 in a high resolution field emission display makes a greater contribution to the pixel or sub-pixel associated with it. This increases the need to be able to control electron emissions and the spread of electron emissions from each emitter 30.
An approach to focusing electrons emitted from the emitter 30 without requiring a separate bias voltage source to bias the focusing electrode is described in U.S. Pat. No. 5,191,217, entitled "Method and Apparatus for Field Emission Device Electrostatic Electron Beam Focussing," issued to Kane et al. This approach makes no provision for modifying the focus parameters in response to the amount of current through the emitter 30.
There is, therefore, a need to provide more reliable control of the spatial distribution of the electrons delivered to the faceplate without causing other problems in field emission displays.